1. Field of the Invention
This invention relates to the field of sulfur dioxide-ethylene-vinyl ester terpolymers, and more particularly, to such terpolymers exhibiting improved thermal stability.
2. Description of the Prior Art
Due to their long, flexible hydrocarbon chains, polyethylenes inherently possess good low temperature behavior, impact resistance, etc., but the very low polarity of these thermoplastics makes them susceptible to attack by such materials as hot hydrocarbon solvents and hot oils. The more polar vinyl ester monomers when copolymerized with ethylene provide resins having improved adhesion to polar substrates and greater resistance to solvents and oils but also result in a diminution of the advantageous properties characteristic of the ethylene homopolymers.
It has been observed that the random introduction of sulfur dioxide into the chain of an ethylene-vinyl ester copolymer increases the polarity of the resin without significantly affecting other physical properties such as stiffness, tensile strength and softening temperature. Thus, U.S. Pat. No. 3,684,778 describes random terpolymers prepared from sulfur dioxide, ethylene and vinyl esters and blends of the terpolymers with other polymers. The terpolymers, which can be blended with antioxidants, ultra-violet light stabilizers, stabilizers, and pigments, can be used for making self-supporting film, cable jacketing and conduit as well as many polyethylene applications.
To date, however, thermal instability of these and other sulfur dioxide-ethylene-vinyl ester terpolymers, often marked by an objectionable odor and a tendency to discoloration, has been a serious obstacle to the wider acceptance and use of these resins. While ethylene-vinyl ester copolymers have a sweet odor owing to the presence of residual unpolymerized vinyl ester, the sulfur dioxide-ethylene-vinyl ester terpolymers possess sulfurous and acetic acid odors which become more pronounced on storage in confined areas at ambient temperature. It has been suggested that sulfinic acid end groups forming during polymerization cause the deacetylation of vinyl acetate groups thereby releasing acetic acid and forming double bonds. The presence of double bonds in the terpolymers makes them prone to undesirable crosslinking and/or discoloration. Sulfur dioxide-ethylene-vinyl ester terpolymers on extended aging at 300.degree. F. turned nearly black in color which became more serious as their sulfur dioxide content increased. Since solvent and oil resistance are optimized at higher sulfur dioxide levels, such aggravated discoloration poses a considerable problem.